The invention relates to a handover method and a call setup method in a mobile communication system that consists of base stations and mobile stations, and in which, at least for some of the mobile stations a mobile station-specific special cell list has been created, which contains the identifier for at least one cell that offers special service to the mobile station. The handover method comprises the following steps: setting at least one handover criterion, measuring the quality and signal level of the radio link in the base station and mobile station, measuring the signal level of the neighboring base stations in the mobile station, transmitting the measurement results acquired by the mobile station from the mobile station to the base station as a report message, and performing the handover to the target cell, when the handover criteria are met. The call setup method comprises the following steps: measuring in the mobile station the signal level of the base station of the camping cell and that of the neighboring base stations, establishing the signaling link between the base station and the mobile station, transmitting the measurement results from the mobile station to the base station as a report message, and performing the call setup in the traffic channel assigned to the mobile station.
Additionally, the invention relates to the mobile station and the arrangement for the selection of the base station.
In cellular mobile communication networks the mobile station can move freely within the mobile communication network and connect to the best base station signal available at the time. Usually all base stations offer essentially identical services to the mobile stations in the network. However, some base stations can be configured to offer a special service to all mobile stations in the network, for example, a special tariff. The base station transmits information about such special service in its broadcast channel so that the subscribers in this region of the network notice the fact and can make use of this service.
The FI patent application publication #970034 describes offering tailored special services from some mobile communication network cells to individual mobile stations or a group of mobile stations by using subscriber-or subscriber group-specific local service regions. Such special services include, for example, special tariffs and data services. In a method according to the publication, a mobile station-specific priority cell list (MPCL) is composed from the selected network cells and the operation of the mobile station is controlled according to this priority cell list. Priority cell information can be stored in the list to be composed by using, for example, CI (Cell Identity) and LAI (Location Area Identifier), or only the CI. An priority cell list consisting of the local service cell data of the mobile station is transmitted to the mobile station MS during location update. On the basis of the priority cell list, the mobile station recognizes the priority cells from which it can receive special services to which the other mobile stations are not entitled. The publication does not describe the use of an priority cell list in base station selection.
FIG. 1 of the attached drawing shows a simplified block diagram of the Pan-European GSM mobile communication system. The MS (Mobile Station) is connected via a radio path to a BTS (Base Transceiver Station), in FIG. 1, to the base station BTS1. An idle mobile station MS receives the broadcast from the base station it has selected. A BSS (Base Station Subsystem) consists of a BSC (Base Station Controller) and base stations BTS controlled by it. An MSC (Mobile Services Switching Center) usually controls several base station controllers BSC. The mobile services switching center MSC is connected to other mobile services switching centers, to a GMSC (Gateway Mobile Services Switching Center), and possibly to an intelligent network IN. Via the GMSC, the GSM network is connected to other networks, such as the PSTN (Public Service Telephone Network), PLMN (another mobile communication network), ISDN network or the intelligent network IN. The operation of the entire system is monitored by the OMC (Operation and Maintenance Center).
The subscriber data of the mobile station MS is permanently stored in the HLR (Home Location Register) of the system and temporarily in the VLR (Visitor Location Register) of the area, in which the mobile station MS is located at the time. The VLR contains the location data for the mobile station MS at the accuracy of the LA (Location Area). The geographic area monitored by the VLR is divided into one or more location areas LA within which the mobile station MS can move freely without notifying the VLR. There can be one or more base stations BTS operating in each location area.
The base stations BTS continuously broadcast information about themselves and their surroundings in their broadcast channel, such as the CI (Cell Identity), information about the neighboring cells, and the location area information LAI. According to the LAI the mobile station MS receiving the broadcast channel from the base station BTS can recognize the location area LA in which the mobile station is located. If the mobile station MS notices, when changing the base station BTS, that the location area information LAI of the base station has changed, the mobile station transmits a location update request into the network.
When moving within the area of the mobile communication network, the mobile station MS normally switches to listen to the base station BTS with the strongest signal. The system tries to establish any mobile station MS originated or terminated calls via this base station BTS. If the base station BTS into which the mobile station MS is connected, cannot provide a traffic channel for the call setup, the mobile station MS attempts the call setup with the base station that provides the next strongest signal. This method is called directed retry
FIG. 2a of the attached drawing shows the signaling of the GSM system between the mobile communication network and the mobile station MS in the establishment of a mobile station MS originated call. When the mobile station MS wants to establish a call, it transmits a signaling channel request Channel_request (message 21) to the network which the network answers by assigning the mobile station MS an SDCCH (Stand alone Dedicated Control Channel) by using the message Immediate_assignment (message 22). The mobile station MS uses the assigned channel to transmit a service request message CM_service_request (message 23) which indicates the quality of the requested service which is, in this case, the call setup in the traffic channel. The authentication of the subscriber for controlling the network access and to prevent different misuses is performed by using the messages Authentication_request (message 24) and Authentication_response (message 25). The authentication is performed by comparing the data stored in the network to the data stored in the mobile station MS. After successful authentication the messages Cipher_mode_command (message 26) and Cipher_mode_complete (message 27) are used to convey data about the ciphering algorithm used for the connection. The messages Setup (message 28) and Call_proceeding (message 29) are used to convey more detailed information about the call to be set up, among other things, the mobile station MS transmits to the network the telephone number which the caller wants to call. The network assigns the mobile station MS a traffic channel for the call by using the message Assignment_command (message 30) which is acknowledged by the mobile station MS by the message Assignment_complete (message 31). At item 32 the mobile station MS receives a line alert tone and when subscriber B answers the call, the connection is established by using the messages Connect (message 33) and Connect_acknowledge (message 34). After this the call is continued in a normal fashion in the traffic channel.
FIG. 2b of the attached drawing shows in the corresponding fashion the call setup signaling of the GSM system in the case of a mobile station MS terminated call. Same reference numbers as in FIG. 2a are used for the messages in FIG. 2b that correspond to the messages explained in FIG. 2a. The network pages the mobile station MS by transmitting the message Paging_request (message 35) of FIG. 2b which the MS reacts to by transmitting the signaling channel request Channel_request (message 21) to the network. The network assigns the MS a signaling channel by using the message Immediate_assignment (message 22) in the same manner as above. The MS uses the assigned signaling channel to transmit a response to the paging message Paging_response (message 36). Subscriber authentication and the data about the ciphering algorithm to be used are exchanged by using messages 24 to 27 as desribed above. Messages Setup (message 37) and Call_confirmed (message 38) are used to initialize the call setup, and messages Assignment_command (message 30) and Assignment_complete (message 31) are used to assign a traffic channel for the mobile station MS for the call. At item 39 the mobile station MS sounds the alert tone and when the mobile station subscriber answers the call, the connection is established by using the messages Connect (message 40) and Connect_acknowledge (message 41). After this, the call is continued in the traffic channel in the normal fashion.
In cellular mobile communication systems the radio coverage is implemented by using several, slightly overlapping radio cells. When the mobile station moves from one cell to another, the system performs a handover to the new radio cell based on predefined handover criteria. The system attempts to perform the handover in a manner that disturbs the ongoing call as little as possible. The handover is normally caused by the criteria of the radio path but it can also be performed for other reasons, such as dividing the load more evenly or to decrease the transmission power. The handover can also be performed within the cell from one traffic channel to another. It is also possible to define a neighbor cell priority list for the base station BTS in which one or some of the neighboring cells of the base station are defined as primary target cells for handover. In this case, the handover is always performed to these priority cells whenever possible. This neighboring cell priority list is base station-specific, and the handover for all mobile stations is performed in the same manner by using it.
The mobile station MS continuously measures the signals of the base stations BTS located nearest to the camping cell, for example, to determine the base station that provides the best signal, and in case of a possible handover. Based on the neighboring cell information broadcast by each base station in its broadcast channel the mobile station MS recognizes which of the neighboring cells it should observe For example, in the GSM mobile communication system the mobile station MS can measure the level and quality of the signal of the serving base station and simultaneously the signal level of up to 32 other base stations. The mobile station MS frequently transmits the measurement results as a report message via the serving base station BTS1 to the base station controller BSC. The report message contains the measurement results of the serving base station and those of up to the six best neighboring base stations. The handover from the current cell to a surrounding cell or to another channel of the current cell may occur, for example, when the measurement results of the mobile station and/or base station controller indicate that the signal level and/or quality of the current cell channel are low and it is possible to obtain a better signal level from one of the surrounding cells, or by switching to another channel, or when one of the surrounding cells/another channel makes it possible to continue traffic by using lower transmission power levels. The selection of the handover target cell is affected, e.g., by the signal level and/or load of the target cell. Therefore, commonly used handover criteria include, for example, the signal level and quality of the radio connection, the signal levels of the current cell and target cell, the signal quality of the current cell, the transmission power required of and allowed for the mobile station in the target cell. The handover from one traffic channel to another or from one signaling channel to another is usually performed when the handover criteria set by the operator are met. It is possible that the handover must be performed because of an overload. Directed retry should also be interpreted as a handover, when the traffic channel that is selected during the call setup is selected from a cell other than that in which the signaling was performed.
FIG. 3 of the attached drawing demonstrates the signaling transmitted in the handover between two base station sub-systems BSS. In the initial situation of the handover in FIG. 3 a radio connection has been established to the mobile station MS, for example, for transmission of voice, data, or signaling, via the base station BTS1 and base station controller BSC1 (not shown in the figure) belonging in base station sub-system BSS1. The base station controller BSC1 receives measurement results about the measurements of the base station BTS1 and mobile station MS and, if necessary, directs a handover from the base station BTS1 to, for example, another base station controlled by the base station sub-system BSS1. When the base station subsystem BSS1 detects the need for a handover to a base station controlled by another base station sub-system, the base station sub-system BSS1 transmits a handover request to the mobile services switching center MSC in the message Handover_required (message 1) which also states the reason for the handover, for example, the quality or signal level of the radio connection, and the identity/identities of the handover target cell. The mobile services switching center MSC transmits the handover request in Handover_request (message 2) to the base station sub-system of the target cell, in FIG. 3 to the base station sub-system BSS2, which states that the radio connection of the mobile station MS will be handed over to the base station sub-system BSS2. The base station sub-system BSS2 responds to mobile services switching center MSC by transmitting the message Handover_request_acknowledge (message 3) which indicates the new radio channel allocated for the mobile station MS. The mobile services switching center MSC transmits the radio channel data to the base station sub-system BSS1 in the message Handover_command (message 4) which is further transmitted by the base station sub-system BSS1 to the mobile station MS in the message Handover_command (message 5). The mobile station MS performs the handover by switching to the new, assigned radio channel, and it acknowledges the successful handover to the base station subsystem BSS2 by transmitting the message Handover_complete (message 6). The base station sub-system BSS2 further transmits the message about successful handover to the mobile services switching center MSC in the message Handover_complete (message 7). The mobile station MS continues the traffic via a base station and a base station controller belonging in the base station sub-system BSS2.
The problem in the currently recognized call setup and handover methods is that the selection of the base station does not support the direction of the mobile station in mobile communication networks that offer the special services tailored in its priority cells but the call setup and handover are performed identically for all mobile station subscribers.
The object of this invention is to implement the base station selection in call setup and handover by taking into account any mobile station-specific special cells.
This new type of handover can be implemented by using a method according to this invention for which it is characteristic that the cell to be assigned to the mobile station is selected on the basis of the measurement results and the special cell list, and the selected cell is set as the target cell for the handover for the mobile station.
Furthermore, the new type of call setup is reached by using a method according to this invention for which it is characteristic that the cell to be assigned to the mobile station is selected on the basis of the measurement results and the special cell list, and the mobile station is assigned a traffic channel from the base station of the selected cell.
Furthermore, the invention relates to the arrangement for selecting the base station for establishing a radio connection in the mobile communication system, said arrangement including the equipment for receiving the measurement results of the mobile station and the equipment for directing the mobile station to the channel of the selected base station, and the mobile station. It is characteristic of the said arrangement, according to the invention, that it includes the equipment for receiving and storing the special cell list, said special cell list including the identifier for at least one cell that offers special service to the mobile station, and the equipment for selecting the base station for the radio connection on the basis of the said special cell list and measurement results. It is characteristic to the mobile station, according to the invention, that it includes the equipment for storing the special cell list, said special cell list including the identifier for at least one cell that offers special service to the mobile station, and the equipment for transmitting the special cell list to the mobile communication network.
The invention is based on the idea that the base station selection is performed on the basis of the special cell list composed for the mobile station When the base station controller directs the selection of the base station, the special cell list of the mobile station subscriber is transmitted to the base station controller during call setup and, if necessary, during handover execution either from the mobile station or from the mobile services switching center. The system attempts to direct the mobile station to its special service area in call setups and handovers. By using the special cell list it is possible to divide the cells of the network into special cells from the view-point of a single mobile station, for example, into priority cells and common cells. In call setup the mobile station is assigned a traffic channel from the base station of a cell defined as a priority cell for the mobile station when the mobile station is located in the area of such a priority cell. The base station of a cell defined as a priority cell for the mobile station is selected as a target cell for handover when the signal of the base station of such a priority cell is suitable for the handover. In an embodiment of the invention, a handover criterion is set which the signal of the base station of the cell defined as a priority cell in the priority cell list must meet in order for the handover from a common cell of the network to the priority cell of the mobile station to be performed.
The advantage of this kind of a selection of the base station performed during handover and call setup is that the mobile station is directed to the special service area and it is kept there whenever it is suitable from the radio technical point of view.
Another advantage of an arrangement according to the invention is that it provides better options for offering individual mobile station subscribers special tailored services preferably throughout the entire call.
The advantage of the mobile station according to the invention is that the functionality according to the invention can be implemented by making minor changes to a mobile station at the prior art.